1. Field of the Invention
The present invention relates to a wheel suspension system for a bicycle and a fork leg therefor.
2. Description of the Related Art
(1) Wheel Suspension System for Bicycle
(A) Conventionally, for bicycles, such as a so-called "mountain bicycle" or "Off-Road bicycle", there is employed a wheel suspension mechanism which includes a sliding tube fixed to a steering bracket which is rotatably supported in the front portion of a bicycle body frame, a pair of left and right cylindrical fork legs slidingly engaged with the sliding tube and supporting a front wheel, and a shock absorber assembly disposed between the sliding tube and a respective one of the fork legs, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-231289, for example. In such a wheel suspension mechanism, when the fork leg slides with respect to the sliding tube, the shock absorber assembly becomes active to absorb road shock caused by undulation on the road surface.
However, in the above-mentioned suspension mechanism, a suspension member for supporting the front wheel with respect to the steering bracket is separated into a sliding tube and the fork legs thus have relatively low rigidity. Particularly, when a brake lever is mounted on the outer periphery of the fork member at the upper portion thereof, a reacting brake force exerted on the brake lever during application of the brake is transmitted to the fork leg as a torsional stress. Therefore, in such case, it becomes necessary to provide sufficient rigidity.
The above-mentioned Japanese Unexamined Patent Publication 2-231289 proposes, as a solution for the lack of rigidity, to provide an essentially a U-shaped cross member with a given torsion strength to connect the left and right fork legs.
However, in such prior art, mounting bosses are necessary in the upper portion of the fork members for connecting the cross member. Typically, a mounting boss is formed by machining, after casing the fork leg with a material, such an as aluminum alloy. This makes it impossible to employ a tube material, such as extrusion tubes, drawing tubes and so forth, as a material for the fork leg.
In view of the problem set forth above, it is an object of the present invention to enable the tube material to be used as the material for the fork legs in constructing the suspension mechanism, in which the sliding tube and the fork legs are telescopically coupled and the upper portions of the left and the right fork legs are joined by means of a cross member.
(B) On the other hand, the conventional shock absorber assembly merely comprises a compression stroke relief valve which is responsive to a shock acting on the suspension mechanism due to an undulation over 1 inch on the road surface to open a compression stroke fluid path, and a check valve which is closed during compression stroke and open during and extension stroke.
With such construction, the conventional shock absorber cannot absorb a small undulation an the load surface and thus causes a rough and uncomfortable ride feeling.
In view of such a defect in the prior art, it is an object of the present invention to provide a wheel suspension system which can satisfactorily absorb shock even when the shock is caused by a substantially small road undulation, for providing a stable and comfortable ride and can generate a damping force both in the compression stroke and the extension stroke.
(2) Fork Leg for a Wheel Suspension System of Bicycle
(C) As the conventional wheel suspension system for the bicycle, the foregoing Japanese Unexamined Patent Publication 2-231289 discloses a structure, in which a damping force generating fluid chambers are defined within the fork legs to slidingly receive therein the bifurcated lower portions of the sliding tube, and a wheel axle supports are formed on the lower reduced diameter sections of the respective fork legs.
In the prior art, such fork legs are manufactured by casting from aluminum alloy and then machining. This increases the manufacturing cost of the fork legs.
The inventors have attempted to manufacture less expensive fork legs by employing a cold forging technique of tubular material.
However, when the fluid chamber is to be defined within the interior space of the fork leg, a substantially high precision level in the shape of the periphery of the fluid chamber is required. For this reason, during a forging process for reducing the diameter at the lower portion of the fork leg, some measure, such as an insertion of a core metal, becomes necessary for maintaining a sufficient level of precision of the shape of the periphery of the fluid chamber. This makes the process complicated and time and labor intensive work.
Therefore, it is a further object of the present invention to make a high precision and less expensive fork legs with a simplified process.